The market for home networking is developing at a phenomenal rate. Service providers from cable television, telephony and digital subscriber line markets are vying to deliver bundled services such as basic telephone service, Internet access and entertainment directly to the consumer. Collectively these services require a high-bandwidth network that can deliver 30 Mbits/s or even higher rates. The Institute of Electrical and Electronic Engineers (IEEE) 802.11a standard describes a cost-effective, robust, high-performance local-area network (LAN) technology for distributing this multimedia information within the home. Networks that will operate in accordance with standard 802.11a will use the 5-GHz UNII (unlicensed National Information Infrastructure) band and may achieve data rates as high as 54 Mbits/s, a significant improvement over other standards-based wireless technologies. The 802.11a standard has some unique and distinct advantages over other wireless standards in that it uses orthogonal frequency-division multiplexing (OFDM) as opposed to spread spectrum, and it operates in the clean band of frequencies at 5 GHz.
OFDM is a technology that resolves many of the problems associated with the indoor wireless environment. Indoor environments such as homes and offices are difficult because the radio system has to deal with a phenomenon called “multipath.” Multipath is the effect of multiple received radio signals coming from reflections off walls, ceilings, floors, furniture, people and other objects. In addition, the radio has to deal with another frequency phenomenon called “fading,” where blockage of the signal occurs due to objects or the position of a communications device (e.g., telephone, TV) relative to the transceiver that gives the device access to the cables or wires of the cable TV, telephone or internet provider.
OFDM has been designed to deal with these phenomena and at the same time utilize spectrum more efficiently than spread spectrum to significantly increase performance. Ratified in 1999, the IEEE 802.11a standard significantly increases the performance (54 Mbits/s vs. 11 Mbits/s) of indoor wireless networks.
The ability of OFDM to deal with multipath and fading is due to the nature of OFDM modulation. OFDM modulation is essentially the simultaneous transmission of a large number of narrow band carriers, sometimes called subcarriers, each modulated with a low data rate, but the sum total yielding a very high data rate. FIG. 1a illustrates the frequency spectrum of multiple modulated subcarriers in an OFDM system. To obtain high spectral efficiency, the frequency response of the subcarriers are overlapping and orthogonal, hence the name OFDM. Each narrowband subcarrier can be modulated using various modulation formats such as binary phase shift keying (BPSK), quaternary phase shift keying (QPSK) and quadrature amplitude modulation QAM (or the differential equivalents). The 802.11a standard specifies that each 20 MHz channel has 52 subcarriers covering 16.5 MHz of the 20 MHz , leaving 3.5 MHz to be used for preventing interference between channels.
Since the modulation rate on each subcarrier is very low, each subcarrier experiences flat fading in multipath environments and is relatively simple to equalize, where coherent modulation is used. The spectrums of the modulated subcarriers in an OFDM system are not separated but overlap. The reason why the information transmitted over the carriers can still be separated is the so-called orthogonality relation giving the method its name. The orthogonality relation of the subcarriers requires the subcarriers to be spaced in such a way that at the frequency where the received signal is evaluated all other signals are zero. In order for this orthogonality to be preserved it helps for the following to be true:                1. Synchronization of the receiver and transmitter. This means they should assume the same modulation frequency and the same time-scale for transmission (which usually is not the case).        2. The analog components, part of transmitter and receiver, are of high quality.        3. The multipath channel needs to be accounted for by placing guard intervals which do not carry information between data symbols. This means that some parts of the signal cannot be used to transmit information.        
The IEEE 802.11a standard defines the structure of a packet that is used for information transmission between two transceivers. A receiver derives timing information, data, and other information from the packet. For example, the first 10 symbols (t1 to t10) in the packet are referred to as the shorts; repeated random sequences that a receiver uses for detecting symbol timing and coarse carrier frequency offset. A guard interval (GI1) follows the shorts and acts as a rough inter-symbol boundary for absorbing the effect of multipath. The guard interval is made long enough such that if short symbol t10 undergoes multipath, symbol t10 will partially “smear” into GI1 without affecting the first long symbol (T1) that follows the shorts. A receiver may receive noise that may cause the receiver to commence processing of the noise as though it were the start of the short symbols. If the receiver fails to detect the false detection relatively quickly, there is the possibility that the receiver will continue to process the noise and fail to process a legitimate packet. U.S. application Ser. No. 09/962,928, filed Sep. 24, 2001, entitled “Detection of a False Detection of a Communication Packet,” describes methods and systems for detecting the false detection of the start of a packet, thereby, allowing the receiver to return relatively quickly to waiting for a legitimate packet.
If the receiver and transmitter are not synchronized as in (1) above, the orthogonality of the subcarriers is compromised and data imposed on a subcarrier may be not be recovered accurately due to inter-carrier interference. FIG. 1b illustrates the effect of the lack of synchronization on the frequency spectrum of multiple subcarriers. The dashed lines show where the spectrum for the subcarrier should be, and the solid lines shows where the spectrum falls due to the lack of synchronization. Since the receiver and transmitter need to be synchronized for reliable OFDM communication to occur, but in fact in practice they do not, it is necessary to compensate for the offset between the receiver and the transmitter. The offset can occur due to the inherent inaccuracy of the synthesizers in the transmitter and receiver and to drift due to temperature or other reasons. The offset can be compensated for at the receiver, but present methods only produce a coarse estimate of the actual offset. According to one method for compensating for the offset, the analog signal received by a receiver is divided into three sections: short symbol section, long symbol section and data symbol section. Some of the short symbols in the short symbol section are used for automatic gain control and for detecting symbol timing. Other short symbols are sampled and digitized and auto-correlated to produce a coarse estimate of the offset. The coarse estimate of the offset is then used to produce a digital periodic signal whose frequency is based on the coarse estimate of the offset.
The digital periodic signal is multiplied with digital samples of the long symbols, when they arrive, and the product is fast Fourier transformed to produce a frequency domain representation of the long symbols as modified by the channel between the transmitter and the receiver (frequency domain representation of received long symbols). The long symbols are a predetermined sequence that is set by the standard to have a predetermined length and information content with a predetermined phase and amplitude. Since the longs are a predetermined sequence, the receiver is designed to store a Fourier transform of the long symbols substantially equivalent to the Fourier transform of the long symbols as transmitted by the transmitter (frequency domain representation of the transmitted long symbols). The quotient of the frequency domain representation of the received long symbols and the frequency domain representation of the transmitted long symbols is the channel estimate or channel transfer function. The channel estimate shows how the channel affects the amplitude and phase of the samples of the long symbols. The inverse of the channel estimate gives an indication of how the samples of a received data signal need to be adjusted in order to compensate for the effect of the channel.
The digital periodic signal is also used to multiply digital samples of the data symbols (digital data samples) when they arrive, thereby correcting for the offset. The data can be recovered from the product of the digital carrier and the digital data samples using the inverse of the channel estimate.
Unfortunately, the inverse of the channel estimate may become invalid with the passage of time due to magnitude changes, frequency offset error, timing drift, and phase noise, and, as such, may be inappropriate to use for decoding data symbols. For example, the pilots of the long symbols on which the inverse channel estimate is based may have an average power magnitude that is different from the average power magnitude of the pilots of a data symbol. Since the 802.11a standard allows transmission using quadrature amplitude modulation, proper decoding of data symbols depends on accurate determination of the amplitude of the subcarriers in a data symbol. Using an inverse channel estimate to decode a data symbol that has pilots whose average power magnitude is different from the average power magnitude of the pilots of the long symbol on which the inverse channel estimate is based may result in improper decoding of the data symbol.
Furthermore, since the short symbols from which the frequency offset was derived are relatively short, the estimate of the offset may be off appreciably from the actual offset. Consequently, there will be a residual offset that may cause the spectrum of one subcarrier to overlap with the spectrum of another subcarrier. Due to the overlap, when data is recovered for one subcarrier, the data may include interference from an adjacent subcarrier, degrading the throughput of the communication system. Furthermore, since there is a residual offset, the channel estimate that was produced using the long symbols is not an accurate representation of the actual transfer function due to the channel. Due to the inaccuracy, errors in data recovery are possible.
Another source of error is timing drift, which causes a data symbol to be sampled earlier or later than specified. Early sampling of a data symbol such that samples of the guard symbol are included in creating a frequency domain representation of the data symbol causes the phases of the subcarriers of the data symbol to rotate by an amount that is proportional to the number of guard samples that sampling is early. If the number of guard samples that sampling is early is known and does not change over time, the effect of the phase rotation can be compensated for. Unfortunately, the number of guard samples that sampling is early drifts over time and needs to be determined in order to compensate for the phase rotation. Another problem with timing drift is intersymbol interference. Sampling for a certain symbol may start early or late causing samples from a previous symbol or a later symbol, respectively, to be used in decoding the certain symbol. This problem is especially evident in multipath environments where much of the guard interval has already been consumed.
Another source of error is phase noise, which affects all subcarrier phases by an equal amount, assuming that the frequency of the phase noise is much less than the symbol frequency. Since the effect of phase noise on the channel estimate is different from the effect on a data symbol, using the channel estimate to decode a data symbol may be inappropriate.
U.S. application Ser. No. 10/076,022, filed Feb. 14, 2002, entitled “An Efficient Pilot Tracking Method For OFDM Receivers,” describes methods and systems that provide a channel estimate that compensates for the factors of frequency offset, timing drift, and phase noise.
It is also desirable to provide alternative techniques to improve the accuracy of inputs to the calculation of such a channel estimate. For example, it would be desirable to provide alternative techniques for correcting for the phase ambiguity when tracking the total change of phase of a signal at a pilot carrier frequency relative to an initial phase value of an initial signal at the pilot carrier frequency associated with one or more training signals.
Consequently, it is desirable to provide a solution that overcomes the shortcomings of existing solutions.